1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a macromolecular-dispersion type liquid crystal display device comprising a liquid-crystal-dispersed layer which comprises a polymer constituting a three-dimensional fine structure and a liquid crystal in the form of droplets dispersed in the polymer. The present invention also relates to a liquid crystal/prepolymer composition which is suitable for the preparation of the above-mentioned macromolecular-dispersion type liquid crystal display device.
2. Discussion of Background
Conventionally, a so-called macromolecular-dispersion type liquid crystal display device is known, which is prepared by holding a liquid-crystal-dispersed layer between a pair of support materials with electrodes. In the liquid-crystal-dispersed layer a liquid crystal in the form of droplets is dispersed in a matrix such as a resin, or a resin three-dimensional network structure is formed in the liquid crystal. The orientation of molecules of the liquid crystal for use in the above-mentioned macromolecular-dispersion type liquid crystal display device is in disorder by the influence of the surrounding matrix with no voltage being applied thereto, so that the light applied to the liquid crystal display device is scattered because of a slight fluctuation of refractive index. When the liquid crystal to be employed has a positive dielectric anisotropy, the molecules of the liquid crystal are oriented in the direction of an electrical field by the application of a voltage to the liquid crystal display device. In this case, the fluctuation of the refractive index of the liquid crystal is reduced, so that the liquid crystal display device assumes a transparent state.
In the case where a dichroic dyestuff is added to the liquid crystal for use in the macromolecular-dispersion type liquid crystal display device, the dyestuff is oriented at random and absorbs the light with no voltage being applied thereto. By the application of a voltage, the dyestuff is oriented perpendicularly to the support materials, and assumes a transparent state.
In any case, a polarizing plate, which is employed in the liquid crystal display device of the twisted nematic mode, is not necessary in the aforementioned macromolecular-dispersion type liquid crystal display device. Therefore, it is possible to clearly display the information in the macromolecular-dispersion type liquid crystal display device without the loss of light, that is, the absorption of light by the polarizing plate. In addition, the macromolecular-dispersion type liquid crystal display device has the advantage of high response speed.
To form such a dispersion structure of the liquid crystal display device, the following methods are proposed:
(1) Emulsion method: an emulsion comprising an aqueous solution of a water-soluble polymer such as polyvinyl alcohol and a liquid crystal is coated on a support material and dried.
(2) Solvent-evaporation method: a homogeneous solution is prepared by dissolving a soluble polymer and a liquid crystal in a solvent, and the solution thus prepared is coated on a support material, and dried so as to carry out the phase separation into a liquid crystal and a polymer.
(3) Photopolymerization method: a mixture of a photopolymerizable material such as an acrylic monomer, a liquid crystal and a photopolymerization initiator is encapsulated in a gap between upper and lower support materials. The encapsulated mixture is irradiated by the ultraviolet light to carry out the photopolymerization of the photopolymerizable material and cause the phase separation.
(4) Heat-polymerization method: a mixture of a heat-polymerizable material such as an epoxy compound, a curing agent therefor, and a liquid crystal is encapsulated in a gap between upper and lower support materials. The heat is applied to the encapsulated mixture to carry out the polymerization of the heat-polymerizable material and cause the phase separation.
The photopolymerization method (3) has been developed actively because the liquid-crystal-dispersed structure in the liquid crystal display device can be controlled, the reproducibility of the performance of the obtained liquid crystal display device is excellent, the thickness of the liquid-crystal-dispersed layer can easily be adjusted, and the application of this method
(3) to the conventional manufacturing process is relatively easy. However, the macromolecular-dispersion type liquid crystal display device prepared by the photopolymerization method (3) has the following drawbacks:
(a) The threshold values are not clear and the operating voltage is high.
(b) The response speed becomes slow when the operating voltage is decreased. The low-voltage driving system and the high-speed response are not satisfied at the same time.
(c) There is a large hysteresis effect in the voltage-transmittance characteristics, so that it is difficult to display a half tone image on the display device.
(d) The charge holding ratio is so low that it is difficult to drive the liquid crystal display device by an active element such as a thin-film transistor.
It is considered that the aforementioned drawbacks (a) to (c) result from inadequate control of the dispersion structure of the liquid crystal, for example, in terms of the shape, the size and the distribution of a liquid crystal phase.